木其滩岩组形成时代、地球化学特征——对北秦岭构造演化的制约

Geochronology and geochemistry of the Neo-Proterozoic amphibolites in the Muqitan Formation: Implications for the tectonic evolution of the North Qinling Orogen

出露于北秦岭造山带中的木其滩岩组主要由斜长角闪岩组成,其原岩为玄武岩-亚碱性玄武岩。对木其滩岩组斜长角闪岩中的锆石进行LA-ICP-MSU-Pb年代学分析,获得同位素年龄762.5±4.6Ma,为成岩年龄。地球化学上,这些斜长角闪岩显示低钛(TiO_2=0.57%～2.16%),低K_2O/Na_2O比值(0.06～0.55)和中等-高Mg~#(45.3～68.7)特征,为低钛拉斑玄武岩。在球粒陨石标准化图解中,表现出轻重稀土分异,无Eu异常的特征,类似于E-MORB。这些斜长角闪岩以Rb、Ba、Th和La富集,低(^(87)Sr/^(86)Sr)i比(0.7038～0.7040)和高的ε_(Nd)(t)值(+4.1～+6.9)为特征,表明其来源于地幔,成分上相似于FOZO。(Th/Nb)_N,Nb/La和Ba/La的比值特征表明它们经历了不同程度的地壳混染。所有这些特征,结合区域地质资料阐明木其滩岩组斜长角闪岩代表的是大约762.5±4.6Ma商丹洋盆初始裂解时的洋壳残片。

The Muqitan ophiolitic mélange found within the Muqitan Formation near Muqitan region, the North Qinling Orogen. They mainly consist of amphibolite with minor meta-gabbro and meta-chert. Zircon LA-ICP-MS U-Pb dating yields an age of 762.5±4.6Ma for the amphibolites, interpreted as crystallization time. The amphibolites are characterized by low TiO2 （0.57%~2.16%）, relatively low K2O/Na2O ratios （0.06~0.55） and moderate-high Mg# （45.3~68.7）. They are geochemically similar to low.Ti tholeiites. The Muqitan amphibolites have low REE contents, with differentiated LREE/HREE patterns and no Eu anomalies on the chondrite-normalized REE diagram, analogies to E-MORB. The amphibolites display an enrich elements Rb, Ba, Th, and La pattern, low （87Sr/86Sr）i ratios of 0.7038 to 0.7040 and high εNd（t） values of ＋4.1 to ＋6.9, suggesting a mantle origin, similar in composition to FOZO-like source. Their （Th/Nb）N, Nb/La and Ba/La ratio features indicate that these amphibolite have been experienced varying degrees of crustal contamination. All these data, combined with the regional geological features demonstrate that the Muqitan amphibolites are interpreted as remnants of the Shangdan ancient oceanic crust that would have formed during the inception of the oceanic open at ca. 762.5±4.6Ma.